DE1571978A1 - Metallic anodes for galvanic high-temperature fuel cells with solid electrolyte and process for their production - Google Patents

Description

Dr. Εχρϊΐ

■ · "

KRU / hu -8/66 February 21, 1966

F-3253-01

Battelle-Institut.e.V., Frankfurt (Main), Wiesbadener Strasse

Metallic anodes for galvanic high-temperature fuel cells with solid electrolyte and methods of making them

The invention relates to metallic anodes for galvanic high-temperature fuel cells with solid electrolyte which conducts oxygen ions and which use gases as fuel and oxidant operate.

The electrochemical oxidation of the fuel takes place. the

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The anode must therefore be in good contact with the electrolyte. In the case of liquid and molten electrolytes, the electrode is wetted by the electrolyte, whereas in the case of solid electrolyte the electrode must adhere well mechanically. The power density of the anode depends on the adhesive surface. The anode materials can be metals or electron-conducting oxides, such as platinum (H. Binder et al., Elektrochimica Acta (London), ίϊ, 781, 1963) or nickel, chromium and cobalt and oxides of titanium and uranium (dT Bray et al. , Chem. Abstr. 6OP, 15443c, 1964).

The known anodes have different structures. They can form thin films that are applied to the electrolyte by burning on pastes containing metal or oxide .

If hydrocarbons are used as fuel gas and thermally decomposed into carbon and hydrogen on the solid electrolyte, the carbon is deposited as a thin layer of graphite on the electrolyte. In this way, an anode is obtained which both establishes contact with the electrolyte and at the same time represents the fuel. A carbon anode only allows the carbon to be oxidized to carbon monoxide, due to its location in the Bourdouard view

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Equilibrium at a working temperature of 800 C and higher (U.S. Patent 3,138,488).

The anode can also be in the liquid state, if a sufficiently high working temperature is used. So can carbonaceous iron melts and liquid, carbon-containing cobalt-tin alloys form the anode (US Pat. No. 3.I38A90).

The anode materials listed have the following disadvantages Platinum is too expensive as an electrode material and, like other metals, does not adhere well to the solid electrolyte; the oxides are also not good anode materials because they are an essential lower conductivity than metallic anodes, which increases the internal resistance of a fuel cell will. In addition, they are not sufficiently chemically stable. Liquid Metal anodes require a high working temperature; this is associated with major corrosion problems; aside from that the cell construction is subject to tight limits. If carbon is used as an anode or fuel, it is used as particular disadvantage to mention the incomplete combustion to carbon monoxide.

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The ancs according to the invention avoid these disadvantages. According to the invention, they have molybdenum and / or olfram. These metal layers have a grain-like structure and therefore suffer no recrystallization phenomena at operating temperature; Adhesion and activity of the invention Anodes are therefore even after a long period of operation unchanged good. The anodes are mechanically very stable. Their porous structure causes good diffusion the fuel gas on the one hand and the combustion products on the other.

However, the invention also relates to a method for the production of the anodes according to the invention. This consists in that the anode metal is mixed with solid electrolyte powder is sintered onto the solid electrolyte.

The production method according to the invention offers the advantage that the solid electrolyte and anode in a single

can be manufactured by operation, whereby / adjustment of the various. Components Tensions between the solid electrolyte and anode are avoided.

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The drawing shows a cooling current density curve (l) of a high-temperature fuel cell, which was recorded at 900 ° C. with a molybdenum anode according to the invention and a silver cathode. On the abscissa is the load on the electrode (mA / cm) on which The associated values of the cell voltage (mV) are plotted on the ordinate. Hydrogen served as the fuel gas, and oxygen as the oxydans. Curve II is the cell characteristic under comparable conditions with conventional platinum anodes. By comparing the curves, it can be seen that the molybdenum anode according to the invention is superior to the platinum anode. This is expressed, for example, in the fact that the polarization that occurs on platinum anodes at low loads does not occur in hard metal anodes, as was also found in tungsten. This avoids a voltage loss, which can be up to 150 mV with platinum. Another

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The advantage of the electrodes is that the metal ^{does not evaporate at the operating temperature of 800 »1000 °} C., as is the case with platinum, for example, since the hard metals molybdenum and tungsten have a very high melting point.

The good conductivity of the electrodes according to the invention should also be emphasized. Ee is even amazing that after that

BATH ORIGINAL

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According to the method according to the invention conductive electrodes at all can be obtained because at the high melting point of the hard metals good sintering of the metal grains - embedded in the electrolyte framework - is hardly to be expected at the temperature to be used for the electrolyte "" The electrodes are even then still conductive, if additional porosity has been created by means of a pore opener «

Their alloys can be used instead of pure metals molybdenum and tungsten, metals such as titanium, iron, cobalt, nickel, chromium and alloys of tungsten and molybdenum use ", but Bs is important that the melting points of the alloys do not below the sintering temperature of the electrolyte IiSgOSi ₉ which can be assumed to be a minimum of 1,600 ° C if zirconium oxide is the basic substance of the solid electrolyte .

The following examples describe the manufacture of anodes according to the invention.

Example T %

To produce the electrolyte , ψ - ^ f of a mixed oxide consisting of 92 mol .- # zirconium oxide with 8 mol .- $ yttrium oxide

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Now filled into a 2k diameter press mold and pressed smooth with very low pressure (approx. 0.1 kp / cm). Around 200 milligrams of an intimate mixture of solid electrolyte powder of the above composition, metal powder and ammonium carbonate are evenly distributed over this layer. The tablet is pressed at a pressure of about 5 Mp / cm. The ammonium carbonate acts as a pore-forming agent, while the oxide powder is intended to anchor the metal particles on the electrolyte. In order for the tablet during sintering at 1800 - can shrink 2000 ⁰ C without tension, the metal content is not possible more than 50 vol .- $> rise in the electrode layer. The metal content of the mixtures varies between 15 and 50 vol .- $, the content of the electrolyte powder is up to 50 vol .- $ and that of the ammonium carbonate up to around 70 vol .- $. The molybdenum electrode, the voltage-current density characteristic of which is shown in Fig. 1 (l), had the following mixture of the electrode layer before sintering:

20 vol .- # molybdenum

30 vol .- # electrolyte powder

50 vol .- $ ammonium carbonate.

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Example 2 ;

The electrode with the electrolyte is made according to Example 1, manufactured but using tungsten instead of molybdenum; the same temperature is maintained during sintering. It has been shown that the following composition of the anode layer before sintering is particularly good Adhesive and functional anode in the sintered state results in:

30 vol .- # tungsten

30 vol .- $ electrolyte powder

50 vol .- $ ammonium carbonate.

Example 3 ;

The finest molybdenum (grain size <5 μm) is ground with electrolyte powder in a ball mill under, for example, butyl acetate, until a fine suspension is created that does not separate. A part of the solvent is removed after sedimentation, whereby a thtcotropic metal paste is formed. This metal paste can be applied to the pre-sintered electrolyte by painting, or it is coated with the metal paste by dipping

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sintered the dense electrolyte. The sintered metal oxide layer conducts the current well if at least TO VdI .- $ Molybdenum are present in the electrode. The oxide of this Metal paste in turn serves to anchor the metal particles on the solid electrolyte. This manufacturing process also produces firmly adhering, porous anodes.

example h :

• According to example 1 or 3, an anode is produced which consists of an alloy of molybdenum with 30 atoms of nickel. The alloy can be used as such according to example 1 or 3 be sintered on the plague electrolyte or the metals are used in a powder mixture for the production of electrodes. If the last-mentioned production method is used, only those metal mixtures can be used, whose Alloy formation at the sintering temperature of the solid electrolyte body runs.

Example 5 ?

According to example k , an anode is produced which consists of an alloy of tungsten with 30 atoms of cobalt

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Alloy can as such according to example 1 or 3 on the plague electrolyte can be sintered or the metals are used in a powder mixture for the manufacture of electrodes used.

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Claims (1)

Pa t e η t a η s ρ r ü c h e 1. Metallic anodes for galvanize tie fuel cells with oxygen-ion conductive solid electrolyte, the at high temperatures with gaseous fuels work, characterized in that the anodes contain metallic molybdenum and / or tungsten. 2. Anodes according to claim T, characterized in that si © made of alloys of molybdenum and / or tungsten with metals, the melting point of the alloys being above the sintering temperature of the electrolyte lies. 3 · Anode according to claim 2, characterized in that the alloy metals iron, cobalt, nickel, chromium, titanium as well as platinum and palladium are. A method for producing the anodes according to claims 1 to 3, characterized in that the anode metal is sintered onto the plague electrolyte in a mixture with plague electrolyte powder. 10 98U / 0 180 5. Procedure oh oh. Claim. 4, characterized » that the metals to be alloyed together on the Plague electrolyte are sintered, whereby the le- entry* 6. Verfaiiren nacii claim, h and 5s characterized in that the sintering takes place under inert gas «, BAD ORKMNAL